This invention relates to a pincushion correction circuit which is useful in a television apparatus.
A side or East-West pincushion correction apparatus is described in copending U.S. Ser. No. 722,600, filed Sept. 13, 1976 for Peter E. Haferl entitled, "PINCUSHION CORRECTION CIRCUIT," in which an impedance is coupled in series with the horizontal deflection coil. A switch is coupled in parallel with the impedance. The switch is operated during the second half of the horizontal retrace interval at times which are progressively advanced and retarded during each cycle of the vertical deflection. The average impedance in series with the horizontal deflection coil is thereby varied at a vertical deflection rate in such a manner as to correct for horizontal or side pincushion distortion. A damping circuit consisting of a resistor is coupled to the impedance network in order to minimize the effect of switching transients and thereby prevent undesirable oscillations in the switched impedance. The resistor may dissipate substantial amounts of power.
In the aforementioned arrangement, pincushion correction is achieved by controlling the amount of energy or current in the deflection winding at the beginning of the trace interval. This in turn is accomplished by controlling the impedance in series with the deflection winding during the second half of the retrace interval. When the average impedance is high, less current tends to flow in the deflection winding and the horizontal trace width is reduced. When the average impedance is low, more current flows in the deflection winding and the trace width increases. The average impedance in series with the deflection winding is controlled by the relative time at which the switch is closed. A smooth, continuously variable pincushion correction requires that the voltage waveform across the impedance network follow the waveform of the retrace pulse without spurious undamped oscillations. Such oscillation will cause the pincushion correction to vary in a manner not continuously related to the time at which the switch is closed.